Porous metal-second phase composites

ABSTRACT

A method is taught for the in-situ precipitation of second phase materials, such as ceramic or intermetallic particles, in a substantial volume fraction of solvent metal matrix. The invention involves the propagating reaction of the second phase-forming constituents in a solvent metal medium to provide a porous composite of finely-dispersed second phase particles in the metal matrix. Exemplary materials include titanium carbide or titanium diboride in an aluminum matrix.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of Ser. No. 132,844, filed Dec. 14, 1987, nowabandoned, which is a division of Ser. No. 927,014, filed Nov. 5, 1986,which is in turn a continuation-in-part of Ser. No. 662,928 filed Oct.19, 1984, now abandoned.

BACKGROUND OF THE INVENTION

The present invention comprises a process for the preparation of porousmetal second phase composite material using an in-situ precipitationtechnique involving a propagating reaction wave which is substantiallyisothermal in the plane of the wave front, and the porous products ofthat process. In one embodiment, a second phase, such as a ceramicmaterial or an intermetallic, is formed directly in a relatively largevolume fraction of metallic or intermetallic solvent matrix, whichsubstantially encapsulates the second phase. The second phase cancomprise a ceramic, such as a boride, carbide, oxide, nitride, silicide,sulfide, oxysulfide or other compound, of one or more metals the same asor different than the solvent matrix metal. Of special interest are theintermetallics of aluminum, such as the aluminides of titanium,zirconium, iron, cobalt, and nickel. In the present invention, thesecond phase is contained in a solvent matrix metal or intermetallic,typically in the form of a porous composite, which can be introducedinto a molten host metal bath to disperse the second phase throughoutthe host metal. Cooling yields a final metal matrix having improvedproperties due to, for example, uniform dispersion of the second phasethroughout the final metal matrix, and fine grain size. Either thesolvent matrix metal or the host metal, or both, may constitute an alloyof two or more metals, and the solvent metal may be the same as, ordifferent than, the host metal. The solvent metal should be soluble inthe host metal, or capable of forming an alloy or intermetallictherewith.

For the past several years, extensive research has been devoted to thedevelopment of metal-second phase composites, such as aluminumreinforced with fibers, whiskers, or particles of carbon, boron, siliconcarbide, silica, or alumina. Metal-second phase composites with goodhigh temperature yield strengths and creep resistance have beenfabricated by the dispersion of very fine (less than 0.1 micron) oxideor carbide particles throughout the metal or alloy matrix of compositesformed, utilizing powder metallurgy techniques. However, such compositestypically suffer from poor ductility and fracture toughness, for reasonswhich are explained below.

Prior art techniques for the production of metal-second phase compositesmay be broadly categorized as powder metallurgical approaches, moltenmetal techniques, and internal oxidation processes. The powdermetallurgical type production of dispersion-strengthened compositeswould ideally be accomplished by mechanically mixing metal powders ofapproximtely 5 micron diameter or less with an oxide or carbide powder(preferably 0.01 micron to 0.1 micron). High speed blending techniquesor conventional procedures, such as ball milling, may be used to mix thepowders. Standard powder metallurgy techniques are then used to form thefinal composite. Conventionally, however, the ceramic component islarge, i.e. greater than 1 micron, due to a lack of availability, andhigh cost, of very small particle size materials, because theirproduction is energy intensive, time consuming and capital intensive.Furthermore, production of very small particles inevitably leads tocontamination at the particle surface, resulting in contamination at theparticle-to-metal interface in the composite, which in turn compromisesthe mechanical properties thereof. Also, in many cases where theparticulate materials are available in the desired size, they areextremely hazardous due to their pyrophoric nature.

Alternatively, molten metal infiltration of a continuous skeleton of thesecond phase material has been used to produce composites. In somecases, elaborate particle coating techniques have been developed toprotect ceramic particles from molten metal during molten metalinfiltration and to improve bonding between the metal and ceramic.Techniques such as this have been developed to produce siliconcarbide-aluminum composites, frequently referred to as SiC/Al or SiCaluminum. This approach is suitable for large particulate ceramics (forexample, greater than 1 micron) and whiskers. The ceramic material, suchas silicon carbide, is pressed to form a compact, and liquid metal isforced into the packed bed to fill the intersticies. Such a technique isillustrated in U.S. Pat. No. 4,444,603 to Yamatsuta et al, herebyincorporated by reference. Because this technique necessitates moltenmetal handling and the use of high pressure equipment, molten metalinfiltration has not been a practical process for making metal-secondphase composites, especially for making composites incorporatingsubmicron ceramic particles where press size and pressure needs would beexcessive and unrealistic.

The presence of oxygen in ball-milled powders used in prior art powdermetallurgy techniques, or in molten metal infiltration, can result in adeleterious layer, coating, or contamination such as oxide at theinterface of second phase and metal. The existence of such layers willinhibit interfacial bonding between the second phase and the metalmatrix, adversely effecting ductility of the composite. Such weakenedinterfacial contact may also result in reduced strength, loss ofelongation, and facilitated crack propagation.

Internal oxidation of a metal containing a more reactive component hasalso been used to produce dispersion strengthened metals, such as coppercontaining internally oxidized aluminum. For example, when a copperalloy containing about 3 percent aluminum is placed in an oxidizingatmosphere, oxygen may diffuse through the copper matrix to react withthe aluminum, precipitating alumina. Although this technique is limitedto relatively few systems, because the two metals must have a widedifference in chemical reactivity, it has offered a possible method fordispersion hardening. However, the highest possible concentration ofdispersoids formed in the resultant dispersion strengthened metal isgenerally insufficient to impart significant changes in properties suchas modulus, hardness and the like.

In U.S. Pat. No. 2,852,366 to Jenkins, hereby incorporated by reference,it is taught that up to 10 percent by weight of a metal complex can beincorporated into a base metal or alloy. The patent teaches blending,pressing, and sintering a mixture of a base metal, a compound of thebase metal and a non-metallic complexing element, and an alloy of thebase metal and the complexing metal. Thus, for example, the referenceteaches mixing powders of nickel, a nickel-boron alloy, and anickel-titanium alloy, pressing, and sintering the mixed powders to forma coherent body in which a stabilizing unprecipitated "complex" oftitanium and boron is dispersed in a nickel matrix. Precipitation of thecomplex phase is specifically avoided.

In U.S. Pat. No. 3,194,656, hereby incorporated by reference, Vordahlteaches the formation of a ceramic phase, such as TiB₂ crystallites, bymelting a mixture of eutectic or near eutectic alloys. It is essentialto the process of Vordahl that at least one starting ingredient has amelting point substantially lower than that of the matrix metal of thedesired final alloy. There is no disclosure of the initiation of anexothermic localized second phase-forming reaction forming a movingisothermal wave front at or near the melting point of the matrix metal.

Bredzs et al, in U.S. Pat. Nos. 3,415,697; 3,547,673; 3,666,436;3,672,849; 3,690,849; 3,690,875; and 3,705,791, hereby incorporated byreference, teach the preparation of cermet coatings, coated substrates,and alloy ingots, wherein an exothermic reaction mechanism forms anin-situ precipitate dispersed in a metal matrix. Bredzs et al rely onthe use of alloys having a depressed melting temperature, preferablyeutectic alloys, and thus do not initiate a moving localized secondphase-forming exothermic reaction at or near the melting temperature ofthe matrix metal.

DeAngelis, in U.S. Pat. Nos. 4,514,268 and 4,605,634, herebyincorporated by reference, teaches reaction sintered cermets having veryfine grain size. The method taught involves the dual effect of reactionbetween and sintering together of admixed particulate reactants that areshaped and heated at temperatures causing an exothermic reaction tooccur, be substantially completed. The reaction products are sinteredtogether to form ceramic-ceramic bonds by holding the reaction mass atthe high temperatures attained. Thus, this reference relates to aproduct with sintered ceramic bonds, suitable for use in contact withmolten metal.

Backerud, in U.S. Pat. No. 3,785,807, hereby incorporated by reference,teaches the concept of preparing a master alloy for aluminum-containingtitanium diboride. The patentee dissolves and reacts titanium and boronin molten aluminum at a high temperature, but requires that titaniumaluminide be crystallized at a lower temperature around the titaniumdiboride formed.

In recent years, numerous ceramics have been formed using a processtermed "self-propagating high-temperature synthesis" (SHS), thatinvolves an exothermic, self-sustaining reaction which propagatesthrough a mixture of compressed powders, generally under externallyapplied pressure, to form dense products. The SHS process involvesmixing and compacting powders of the constituent elements, and locallyigniting a portion of a green compact with a suitable heat source. Thesource can be electrical impulse, laser, thermite, spark, etc. Onignition, sufficient heat is released to support a self-sustainingreaction, which permits the use of sudden, low power initiation of hightemperatures when using relatively low concentrations of binder, ratherthan bulk heating over long periods at lower temperatures. Exemplary ofthese techniques are the patents of Merzhanov et al, U.S. Pat. Nos.3,726,643; 4,161,512; and 4,431,448 among others, hereby incorporated byreference.

In most SHS processes, the product is a ceramic, that may be relativelydense for use as a finished body, or may be crushed for use as a powderraw material. In a few instances, binders, such as metal, have beenincluded with the compressed powders, but typically constitute 10percent or less by weight of the mixture, and almost invariably lessthan 30 percent. At these levels, the binder acts as a ductileconsolidation aid to fill in porosity during the exothermic ceramicproduction reaction, and to increase the product density. The denseproducts according to the teachings of the Merzhanov et al. patents arerestricted to binder concentrations below about 30 percent by mass, topreserve wear-resistance and hardness, and porosities below 1 percent toavoid impairing operating performance. Further, the SHS process, even inthe presence of metal, occurs at higher temperatures than those employedin the present invention, and is not isothermal as is the presentinvention because significantly lwoer metal concentrations are employed.Thus, the SHS process yields sintered ceramic particles, havingsubstantial variation in size.

In U.S. Pat. No. 3,726,643, there is taught a method for producinghigh-melting refractory inorganic compounds by mixing at least one metalselected from Groups IV, V, and VI of the Periodic System with anon-metal, such as carbon, boron, silicon, sulfur, or liquid nitrogen,and heating the surface of the mixture to produce a local temperatureadequate to initiate a combustion process. In U.S. Pat. No. 4,161,512, aprocess is taught for preparing titanium carbide by ignition of amixture consisting of 80-88 percent titanium and 20-12 percent carbon,resulting in an exothermic reaction of the mixture under conditions oflayer-by-layer combustion. These references deal with the preparation ofceramic materials, absent a binder.

More particularly, U.S. Pat. No. 4,431,448 teaches preparation of adense, hard alloy by intermixing powders of titanium, boron, carbon, anda Group I-B binder metal or alloy, such as an alloy of copper or silver,compression of the mixture, local ignition thereof to initiate theexothermic reaction of titanium with boron and carbon, and propagationof the ignition, resulting in an alloy comprising titanium diboride,titanium carbide, and up to about 30 percent binder metal. Uponcompletion of the exothermic reaction the resulting solid-liquidreaction mass is subjected to compression until a porosity of below 1percent is obtained. This reference, however, is limited to the use ofGroup I-B metals and alloys, such as copper and silver, as binders. Asmentioned, products made by this method are dense and concentration ofthe binder is restricted to less than 30 percent to preserve wearresistance and hardness.

Several intermetallic self-sustaining reactions have been studiedtheoretically to determine propagation rates, as reported in thefollowing two articles: A. P. Hardt and P. V. Phung, Propagation ofGasless Reactions in Solids--I. Analytical Study of ExothermicIntermetallic Reaction Rates, Combustion and Flame 21, 77-78 (1973); andA. P. Hardt and R. W. Halsinger, Propagation of Gasless Reactions inSolids--II. Experimental Study of Exothermic Intermetallic ReactionRates, Combustion and Flame 21, 91-97 (1973). Compressed shapes werestudied with some binder to cohere the compact. Experimentationconcerned exothermic condensed phase reactions and suggested thedesirability of low heat transfer to permit heat accumulation in thereaction zone in order to allow reaction propagation. Small particlesize reactants were also said to be desirable to permit a high rate ofmass transfer, which allows the reaction to go to completionspontaneously. Thus, heat capacity, heat of reaction, and particle sizewere reported to be important factors. Results showed that increasedconcentrations of binder were undesirable, particularly concentrationsexceeding 30 percent by weight, which retarded ignition and prolongedpropagation, contrary to the present invention.

U.S. Pat. No. 4,540,546 to Giessen et al, hereby incorporated byreference, teaches a method for rapid solidification processing of amultiphase alloy. In this process, two starting alloys react in a mixingnozzle in which a "Melt Mix Reaction" takes place between chemicallyreactable components in the starting alloys to form submicron particlesof the resultant compound in the final alloy. The mixing and chemicalreaction are performed at a temperature which is at or above the highestliquidus temperature of the starting alloys, but which is alsosubstantially below the liquidus temperature of the final alloy, and asclose to the solidus temperature of the final alloy as possible. Whiledispersion-strengthened alloys can be produced by this technique, thereappear to be a number of inherent difficulties. First, processing istechnically complex, requiring multiple furnaces. Second, efficientmixing is important if fine dispersions are to be consistently produced.Lastly, very high degrees of superheat will be required to completelydissolve the rapid solidification alloying elements in order to producehigh loading of dispersoid, which necessarily accentuates particlegrowth, for example, in composites containing 10-20 percent dispersoid.

The present invention overcomes the disadvantages of the prior art. Moreparticularly, the present invention permits simplification of proceduresand equipment compared to the prior art. For example, the presentprocess obviates need for multiple furnaces and mixing and controlequipment because all of the reactive constituents of the second phaseare present in a single reaction mass, in the presence of largeconcentrations of solvent metal. The present invention also overcomesthe need for forming multiple melts of components at very high meltingtemperatures. Further, high loading composites can be prepared withoutthe necessity of achieving high levels of superheat in holding furnaces.Applicants' invention also provides for a cleaner particle/metalinterface compared with conventional metal-ceramic composites made bytechniques using, for example, separate metal and ceramic powders,because the reinforcing particles are formed in-situ and encapsulatedwith solvent metal. Moreover, the porous products formed can bedissolved to make uniform dispersions of substantially unagglomeratedparticles in a matrix, with controlled volume fractions of second phasematerials. With these facts in mind, a detailed description of theinvention follows, which achieves advantages over known processes.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an inexpensivemethod for forming porous composite materials, containing finelydispersed second phase, such as a particulate ceramic, intermetallicmaterial, or mixtures thereof, in metal, metallic alloy, orintermetallic matrices.

The present invention produces a porous composite comprising a reltivelyconcentrated second phase dispersion in a solvent metal matrix, whichmay be the same or different than the final metal matrix desired. Thisconcentrated composite may be utilized to form improved final metalmatrix composites of lower second phase concentration, havingsubstantially uniform dispersion and uniform particle size distribution,by admixture with a molten bath of the desired host metal, metal alloyor intermetallic matrix material, or by admixture with solid host metal,metal alloy, or intermetallic, followed by heating to a temperatureabove the melting point of the host metal.

For purposes of simplifying further description, the matrix of theporous composite material produced directly by the method of the presentinvention shall be referred to as the "solvent metal matrix," while themetal with which the porous composite may be admixed shall be referredto as the "host metal." The metal of the final composites resulting fromsuch admixture may be referred to as "final metal matrix". In eachinstance, the word "metal" shall encompass the alloys and intermetalliccompounds thereof. Further, the solvent metal may encompass not onlymetals in which the second phase-forming constituents are soluble, butalso such metals in combination with other metals. Other metals mayinclude those in which said constituents are not soluble, but in whichsaid solvent metal is soluble, or which are soluble in said solventmetal. Thus, "solvent metal" may refer to a combination of solventmetals and nonsolvent metals.

It is a further object of this invention to provide a method fordispersion hardening of metals and alloys. The present invention relatesto the preparation of a porous composite, said porous compositecomprising a second phase particulate in a solvent metal matrix, such astitanium diboride or titanium carbide in an aluminum matrix, using amoving, substantially isothermal, wave front effecting localized in-situprecipitation of second phase material in relatively large volumefractions of solvent metal to make porous products which can be used toform materials having substantially uniform distribution of second phasematerial. The term "relatively large volume fractions of solvent metal"as used herein shall refer to the presence of at least 10 percentsolvent, preferably more than 20 volume percent, and most preferablymore than 30 percent solvent metal. When the volume percentage of secondphase material exceeds 70 percent, the second phase particles willgenerally be in contact with each other purely by geometricconsiderations, i.e. there is less than 30 volume percent free space ina close-packed array of dispersoids. Accordingly, the incidence ofinterparticle sintering increases substantily as the volume fraction ofthe second phase increases above 70 volume percent in some systems.Thus, it is preferred that the second phase constitute less than about70 volume percent of the composite.

The present invention relates to a process for the localized in-situprecipitation of up to less than about 90 percent by volume of a secondphase material in a solvent metal matrix, wherein the second phase cancomprise a ceramic, such as a boride, carbide, oxide, nitride, silicide,oxysulfide, or sulfide of a metal the same as or other than the solventmetal matrix. It has been found that by mixing the constituents orelements of the desired second phase material with a solvent metal, andlocally heating to a temperature at which substantial diffusion and/ordissolution of the reactive elements into the solvent metal can occur,typically at or close to the melting point of the solvent metal, amoving localized solvent assisted isothermal reaction, which is alwaysexothermic, can be initiated. This solvent assisted reaction results inthe extremely rapid formation and dispersion of finely divided particlesof the second phase material in relatively high concentrations of thesolvent metal matrix material.

It is an object of the present invention to provide a process forforming metal-second phase composite materials having a relativelyuniform dispersion of second phase particulate throughout large volumesof solvent matrix metal. The process comprises localized ignition tocause in-situ precipitation of at least one second phase material in asolvent metal matrix by contacting reactive second phase-formingconstituents, in the presence of a solvent metal, at a localizedtemperature at which sufficient diffusion of the constituents into thesolvent metal occurs locally to initiate a moving isothermal reaction ofthe constituents to produce a porous composite material.

It is also an object of the present invention to provide a method forthe production of porous metal-second phase composite material, themethod comprising precipitating at least one second phase material in asubstantial volume fraction of solvent metal by locally ignitingreactive second phase-forming constituents, in the presence of asubstantially nonreactive solvent metal in which the secondphase-forming constituents are more soluble than the second phasematerial, at a temperature at which sufficient diffusion of the reactivesecond phase-forming constituents into the substantially nonreactivesolvent metal occurs to cause a substantially isothermal propagatingsecond phase-forming reaction of the constituents, to therebyprecipitate second phase particles in the solvent metal so as to producefinely divided second phase particles in the solvent metal matrix.

The invention further relates to a method for the production of porousmetal-second phase composite materials, the method comprisingprecipitating at least one seocnd phase material in a substantial volumefraction of solvent metal by locally igniting reactive secondphase-forming constituents, in the presence of a substantiallynonreactive solvent metal in which the second phase-forming constituentsare more soluble than the second phase, at a local temperature at whichsufficient diffusion of the constituents into the solvent metal occurs,to cause a substantially isothermal propagating reaction of the reactivesecond phase-forming constituents to increase the temperature to atemperature exceeding the melting temperature of the solvent metal, toprecipitate the second phase in the solvent metal matrix.

The invention further relates to a method for dispersion of second phasedispersoids in a metallic matrix, the method comprising forming areaction mixture of reactive second phase-forming constituents in thepresence of a substantial volume fraction of at least two metals, atleast one of which acts as a solvent metal in which the secondphase-forming constituents are more soluble than the second phasedispersoids, raising the temperature of the reaction mixture locally toa temperature at which sufficient diffusion of the second phase-formingconstituents into the lowest melting solvent metal occurs to initiate asubstantially isothermal reaction of the constituents, whereby theexothermic heat of reaction of the constituents causes the temperatureof the reaction mixture to exceed the melting point of the highestmelting metal, permitting propagation of the reaction and dispersion ofthe second phase dispersoid in a metal matrix.

The invention further relates to a method for dispersion of second phasedispersoids in a solvent metal matrix, the method comprising forming areaction mixture of reactive second phase-forming constituents in thepresence of a substantial volume fraction of at least two metals, atleast one of which acts as a solvent metal in which second phase-formingconstituents are more soluble than the second phase dispersoids, raisingthe temperature of the reaction mixture locally to a temperature atwhich sufficient diffusion of the second phase-forming constituents intothe lowest melting solvent metal occurs to initiate a substantiallyisothermal reaction of the constituents, whereby the exothermic heat ofreaction of the constituents causes the temperature of the reactionmixture to exceed the melting point of the lowest melting point metal,permitting propagation of the reaction and dispersion of the secondphase dispersoid in a mixed metal matrix.

The invention further relates to a method for dispersion of at least oneintermetallic material in a metallic matrix.

The invention further relates to a method for dispersion of at least oneceramic material in a metallic matrix.

The invention further relates to a method for dispersing dispersoidparticles of an intermetallic material and a ceramic material in ametal, metal alloy, or intermetallic matrix.

The invention further relates to a porous mass comprising a dispersionof in-situ precipitated insoluble second phase particles in a solventmetal matrix produced by propagating a locally ignited substantiallyisothermal exothermic reaction of second phase-forming constituents inthe presence of a substantial volume fraction of solvent metal in whichthe constituents are more soluble than the second phase.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a novel technique for preparing usefulmetal-second phase composites. Novelty resides in the process forpreparing a porous solvent metal matrix-second phase master concentratesuitable for use as an intermediate in the formation of a densecomposite product. Because the process relies upon the production ofsecond phase particles which are dispersed and substantially insolublewith respect to the solvent matrix metal, the porous composite matrixcan be dissolved in another metal to yield dense composite productshaving a uniform second phase dispersion. Resultant composites may beremelted, facilitating subsequent processing. The technique comprisesthe preparation of a master porous composite containing discretedispersoid particles each substantially encapsulated or enveloped bysolvent matrix metal. Thus, the discrete dispersoid particles are notbonded to each other in the concentrate. This novel technique reliesupon substantial concentrations of high thermal conductivity solventmatrix metal to establish an isothermal wave front which propagates froma place of local ignition of reactive components. More particularly, agreen compact of compressed reactive components, typically shaped in theform of a rod, is ignited at one end, and the substantially isothermalwave front moves along the rod to precipitate the substantiallyinsoluble second phase material in-situ in the relatively highconcentration solvent metal to form the aforementioned porous composite.

An advantage of the present invention is that such porous compositesmay, in turn, be utilized via an admixture process to introduce thesecond phase into a host metal in controlled fashion. Thus, aconcentrate may be prepared in the form of a porous composite having,for example, a high percentage of a second phase, such as a ceramic,e.g. titanium diboride, in a solvent matrix metal, such as aluminum.This porous composite may then be added to a molten host metal, metalalloy or intermetallic bath, (which molten metal may be the same ordifferent from the matrix metal of the porous concentrate) to achieve afinal composite having the desired loading of second phase.Alternatively, the porous composite may be admixed with solid hostmetal, metal alloy or intermetallic, and then heated to a temperatureabove the melting point of the host metal. In the following discussion,admixture with a "host metal" or "host metal bath" should be understoodto apply equally to each of the different embodiments indicated above.

The melting point of the solvent metal must be below the temperature ofthe host metal, and there must be sufficient miscibility of the twomolten metals to insure alloying, dissolution, or combination. Forexample, titanium can be reinforced by precipitating titanium diboridein aluminum, and subsequently introducing the titanium diboride-aluminumcomposite into molten titanium to dissolve the aluminum matrix of theporous composite, thus forming an aluminum-titanium matrix havingtitanium diboride dispersed therein. Similarly, lead can be reinforcedby precipitating titanium diboride in aluminum and admixing thecomposite with molten lead.

In certain instances, the "host metal" may comprise material other thanconventional metals, metal alloys or intermetallics. The host metal may,for example, be a dispersion strengthened metal such as metal containingfinely dispersed erbium oxide, thoria, alumina, etc., or a metal-secondphase composite. It is important in these cases that the preexistingdispersion be stable in the molten metal for the time/temperaturerequired for introducing the desired porous composite material of thepresent invention. The advantage of utilizing a material containing asecond phase dispersion as the host metal is that a bimodal distributionof second phase types, shapes, amounts, etc. may be obtained. An examplewould be the use of an aluminum matrix containing a dispersion ofessentially equiaxed TiB₂ particles, to which a porous composite of thepresent invention dispersed therein having needle shaped TiN particlesis added. A combination of dispersion strengthening and high temperaturecreep resistance is obtained. In accordance with the foregoingdiscussion, it must be understood that suitable "host metal," or "hostmetal, metal alloy or intermetallic" matrices encompass the types ofmaterials discussed above containing preexisting second phasedispersions.

The present invention encompasses several features that run directlycontrary to the prior art wisdom in materials science, and particularlyin the field of metal-second phase composites. These features may bemore clearly understood when considered in the context of the prior artSHS processes, particularly those SHS processes employing minor amountsof binder metal. Firstly, this prior art is directed to the formation ofdense products. Porous products are not considered desirable and havenot been investigated. The perspective of the prior art is to developfinished dense products, i.e. products that are ready for machining orother metal working or ready for use in fabricating manufactures.

Secondly, the prior art looks to the use of a metal matrix as a binder.Contrary to the prior art, the present invention looks to the metalmatrix as a solvent in which dispersoid particles that are substantiallyinsoluble relative to the solvent metal matrix are dispersed. Moreparticularly, the prior art utilizes relatively low concentrations ofbinder, whereas the present invention utilizes relatively highconcentrations of metal matrix. Prior art materials utilize quantitiesof metal which fill voids among typically sintered ceramic particles todensify the composite.

Furthermore, prior art concentrations preclude encapsulating thedispersoid particle with binder. The concentrations utilized in thepresent invention, however, are sufficient to provide for substantialenveloping or encapsulation of the substantially insoluble dispersoidparticles in the solvent metal matrix. This feature is advantageous overprior art in which preformed ceramic powders are combined with metals,because it inhibits the formation of deleterious coatings or layers onthe particles. These coatings or layers, such as oxide layers, arefrequently present in prior art metal matrix composites, and arebelieved to negatively affect physical properties of materials andinhibit further processing of products formed therefrom.

Next, the prior art seeks dense products involving self-bonding ofceramic particles. For example, prior art techniques seek sintering ofparticles, rather than encapsulation thereof to inhibit bonding of theparticles. These kinds of irreversible techniques are directly contraryto the present invention, which utilizes a reversible technique, thatis, an encapsulation technique which inhibits bonding of dispersoidparticles and facilitates further processing.

Additionally, the higher concentrations of metal matrix, typicallygreater than about 30 volume percent, utilized in the present inventionenhance the heat transfer characteristics of reactant combinations andcause a more uniform linear reaction rate. Additionally, there is areduction in particle size because the maximum temperature attained islower than attained in the prior art because of the additional heatcapacity of the contained metal, and because of the more rapid quenchrate resulting from the higher thermal conductivity of said metal.Another advantage is in spatial temperature uniformity, hence uniformityin size distribution of the dispersoid particles in the matrix material.Prior art techniques result in larger particles that are agglomeratedand/or sintered. The smaller particle size and uniformity indistribution of particles achieved by the present invention result inimproved properties of final composite products.

Another feature of the present invention which distinguishes from theprior art is the formation of an isothermal wave front which promotesthe uniformity of particle size of dispersoid particles in a crosssection of the product produced. The isothermal character results fromthe selection of a high thermal conductivity solvent metal matrix, incombination with concentrations of the solvent metal sufficient toachieve the isothermal character across the material to be reacted.

The combination of these features permits manufacture of compositematerials suitable for processing via the admixture process to producematerials whose properties may be tailored to suit the demands ofparticular uses. This admixture process takes advantage of the fact that"poor quality" intermediate composites are recovered. Such compositeswould have heretofore been regarded as useless. For example, in thepreparation of ceramic bodies by SHS, a limiting feature in the processas a means for producing useful ceramic shapes or parts, has been theinherently poor physical quality of the body typically formed by theself-propagating synthesis. Accordingly, attempts have been made toenhance the quality of such bodies by techniques such as elevatedpressures at temperature to cause diffusion, sintering, anddensification. In contrast, such properties as friability, low strength,and porosity have been found, surprisingly, to be advantageous in theprocess herein disclosed.

A feature in the admixture process is that molten metal may be used toadvantage in the production of composites, even though it is well knownin the art that molten metal should be specifically avoided in thefabrication and utilization of metals, ceramics and composites. Thus,for example, the infiltration of molten metals into conventionalpolycrystalline metals results in grain boundary dehesion, facilitatescrack propagation, and hence causes brittleness. As a consequence, therehave traditionally been problems, for example, with the containment ofmolten metal in metallic containers (of higher melting point) because ofprogressive loss of strength and integrity (the phenomenon of liquidmetal embrittlement). Similarly, in the use of ceramics in molten metalcontacting applications, service longevity has always been a problemowing to molten metal attack, even with the most chemically inert andresistant materials. Thus, for example, the containment of moltenaluminum by titanium diboride has been a long standing, and stillcommercially unresolved problem, owing to penetration of the moltenmetal along the ceramic grain boundaries where reaction takes place withcontaminants. Progressive penetration and reaction ultimately lead toloss of intergranular cohesion, mechanical weakness, and disintegration.

The presence of molten metal is equally disadvantageous in themanufacture and use of metal-second phase composites, where it has beenregarded of paramount importance to avoid the introduction of moltenmetal. Several examples are known to illustrate the type of problemsthat can arise. In the preparation of composites of SiC in Al,precautions must be taken, such as proprietary coating techniques, toavoid prolonged direct contact of the molten metal and particulate (orthe ceramic skeleton in the case of molten metal infiltration). Absentsuch precautions, the metal and ceramic react together, a process thatobviously diminishes the amount of particulate reinforcement, but alsogenerates reaction products that may render the composite extremelysusceptible to subsequent corrosion. Analogous problems occur whenattempts are made to weld the SiC/Al because, as the melting temperatureof the matrix metal is exceeded, the same harmful reactions occur. Inthe case of thoria-dispersed (TD) nickel, the composite is produced viasolid powder metallurgical techniques, as opposed to liquid metal (ingotmetallurgy), because the thoria ceramic tends to segregate, and evenrise to the surface of the melt, because of surface tension effects. Aswith SiC/Al, welding is again a problem because of the presence ofliquid metal, this time giving rise to the above-noted segregation.

It would thus be expected that the combination of poor qualitymetal-second phase preforms with molten metal would not lead to therecovery of a useful product. However, it has surprisingly been foundthat employing these features in the admixture process invention yieldsunexpected and quite unobvious benefits, yielding products that hadheretofore been unattainable using prior art techniques.

In addition to the novel and beneficial processing features alluded toabove, other advantages derive from the isothermal, propagating in-situsecond phase precipitation process of the present invention, such asclean coherent interfaces between the metal and second phase. Moreover,the admixture process allows these advantages to be achieved, whileavoiding the shortcomings below, that are inherent to in-situprecipitation of a second phase in metal. Thus, for the production offine precipitates, the process must, by necessity, avoid prolongedheating at elevated temperatures, which results in particle growth. Forthis reason, relatively high concentrations of dispersoid precursor arepreferred in order that the brief duration of exothermic heat besufficient to complete the in-situ formation process. In the case ofhigher concentrations, excessive heat is experienced, hence sinteringand agglomeration of particles results. In the case of lower dispersoidconcentrations, the amount and time of external heat that must beapplied to complete the reaction are such that particle growth may be aproblem. Thus, the range of second phase loadings that may be recoveredin a product is constrained by these criteria. However, when theadmixture process is used, the constraint disappears because theparticle formation process may be conducted under the circumstances thatmost effectively lend themselves to the production of second phase ofthe desired morphology, size, type and other characteristics, withoutregard to loading level. As an example, the optimum second phase loadingrange may be used in the initial propagating isothermal second phaseformation process. This preformed porous composite may then be combinedwith molten host metal in variable amounts, to provide full latitude indispersoid concentration in the recovered final metal matrix.

The present invention is directed to a novel process for the in-situprecipitation of fine particulate second phase materials, such asceramics or intermetallics, typical of which are refractory hard metalborides or aluminides, within metal, alloy, and intermetallic systems,to produce a solvent metal-second phase composite suitable for use as amaster concentrate in the admixture process. However, the processdescribed may also be used for introducing larger particles of a secondphase material into molten host metal, up to the point at which suchlarger particles result in component embrittlement, or loss ofductility, etc. The improved properties of the novel final compositesoffer weight-savings in stiffness limited applications, higher operatingtemperatures and associated energy efficiency improvements, and reducedwear in parts subject to erosion. A specific use of such material is inthe construction of turbine engine components, such as blades.

In this context, it should be noted that the final metal-second phaseproducts of the present invention are also suitable for use as matrixmaterials, for example, in long-fiber reinforced composites. Thus, forexample, a particulate reinforced aluminum composite of the presentinvention may be used in conjunction with long SiC or carbon fibers toenhance specific directional properties while retaining high transversemodulus. Typical fabrication routes for such materials include diffusionbonding of thin layed-up sheets, and molten metal processing.

A method is taught whereby the second phase forming elements are causedto react in a solvent metal to form a finely-divided dispersion of thesecond phase material in the solvent metal matrix. In accordance withthe present invention, the second phase-forming constituents most easilycombine at or about the melting temperature of the solvent metal, andthe exothermic nature of this reaction causes a very rapid temperatureelevation or spike, which can have the effect of melting additionalmetal, simultaneously promoting the further reaction of the secondphase-forming constituents.

In systems where the reactive elements have substantial diffusivity inthe solid matrix metal, the reaction may be initiated at temperatureswell below the melting point of the matrix metal. Thus, a solid stateinitiation is possible, wherein a liquid state may or may not beachieved.

Exemplary of suitable second phase ceramic precipitates are the borides,carbides, oxides, nitrides, silicides, sulfides, and oxysulfides of theelements which are reactive to form ceramics, including, but not limitedto, transition elements of the third to sixth groups of the PeriodicTable. Particularly useful ceramic-forming or intermetalliccompound-forming constituents include aluminum, titanium, silicon,boron, molybdenum, tungsten, niobium, vanadium, zirconium, chromium,hafnium, yttrium, cobalt, nickel, iron magnesium, tantalum, thorium,scandium, lanthanum, and the rare earth elements. Particularly usefuladditional intermetallic-forming elements include copper, silver, gold,zinc, tin, platinum, manganese, lithium and beryllium. Preferred secondphase materials include titanium diboride, titanium carbide, zirconiumdiboride, zirconium carbide, zirconium disilicide, and titanium nitride.

As the solvent metal, any metal capable of dissolving or sparinglydissolving the constituents of the second phase, and having a lessercapability for dissolving the second phase precipitate may be used.Thus, the solvent metal component must act as a solvent for the specificreactants, but not for the desired second phase precipitate. The solventmetal acts primarily as a solvent in the process of the presentinvention, and the constituents of the second phase precipitate have agreater affinity for each other than either has for the solvent metal.Additionally, it is important that the second phase-forming reactionreleases sufficient energy for the reaction to go substantially tocompletion. While a large number of combinations of matrices anddispersoids may be envisioned, the choice of in-situ precipitated phase(ceramic or intermetallic) in any one given matrix, is limited by thesecriteria.

Suitable solvent matrix metals include aluminum, nickel, titanium,copper, vanadium, chromium, manganese, cobalt, iron, silicon,molybdenum, beryllium, silver, gold, tungsten, antimony, bismuth,platinum, magnesium, lead, zinc, tin, niobium, tantalum, hafnium,zirconium, and alloys of such metals.

The host metal may be any metal in which the second phase precipitate isnot soluble, and with which the second phase does not react during thetime/temperature regime involved in the admixture process, subsequentfabrication, and/or recasting. The host metal must be capable ofdissolving or alloying with the solvent metal, and must wet the porouscomposite. Thus, the host metal may be the same as the solvent metal, analloy of the solvent metal, or a metal in which the solvent metal issoluble. When alloys are utilized, one may substantially retain thebeneficial properties of the alloys, and increase, for example, themodulus of elasticity, high temperature stability, and wear resistance,although some loss of ductility may be encountered in certain softalloys. Further, final metal matrix composites prepared from the solventmetal matrix materials of the present process may be fabricated inconventional fashion, by casting, forging, extruding, rolling,machining, etc., and may also be remelted and recast while retainingsubstantial uniformity in second phase particle distribution andretaining fine second phase particle size, fine grain size, etc.,thereby maintaining associated improvements in physical properties.

The degree of porosity of the porous composite can be varied byprocedures such as vacuum degassing or compression applied prior to,during, or subsequent to initiation of the second phase-formingreaction. The degree of vacuum applied and temperature of the degassingstep is determined purely by the kinetics of evaporation and diffusionof any absorbed moisture or other gases. High vacuum and elevatedtemperatures aid the degassing operation. In the case of titanium,aluminum, and boron mixtures, however, the pre-reacted compact must notbe exposed to temperatures above 300° C. for prolonged periods of time,as this will induce the volatilization of some components and induce theformation of titanium aluminide by solid state diffusion. This isundesirable because it forms as large plates, which are detrimental tomechanical properties, and also reduces the chemical driving force forthe formation of the titanium diboride. Nonetheless, conversion oftitanium aluminide to titanium diboride in the presence of boron andaluminum can occur slowly if the components are held at temperaturesabove the melting point of aluminum.

When vacuum degassing is applied prior to reaction, lower porosity isobtained. When vaccum is applied during reaction, the compact typicallyexpands, resulting in a significant increase in porosity.

Absent the degassing step, the composite formed may be relativelyporous, and lower in density than the matrix metal. In such a state,this material may be of a high second phase concentration and may beadded to a measured volume of matrix metal (either the same or differentfrom the matrix in which the dispersoid was first formed) to achieve aspecifically desired second phase volume fraction. Relatively highconcentration of the second phase in the solvent metal matrix may beachieved while retaining substantially uniform dispersion of discretesecond phase particles within the solvent metal matrix.

In preparing the porous composite materials, degassing of the powders ofthe reactant mixture may not be necessary, and, in fact, it may beadvantageous not to degas the powders, because a porous product tends tobe advantageous in the subsequent addition to a host metal. It may evenbe desirable, in some instances, to incorporate a porosity enhancer suchas a low boiling point metal, for example, magnesium in the initialreactant mixture, the enhancer volatilizing during the in-situ reaction,thereby increasing the porosity of the resultant composite.

As formed, the second phase particles of the porous composite areprotected from oxide or other deleterious covering layers which form onprior art ceramic powders. The in-situ formed second phase, such asceramic, of the present invention, uniformly dispersed within a solventmatrix metal, may be introduced into a molten host metal bath toredisperse the second phase particles of the porous composite throughoutthe host metal. The molten host metal of the bath may be of suchcomposition that in-situ precipitation of the desired second phase couldnot occur within the bath, or could occur only with difficulty. Thus,metals other than the solvent-matrix metal may be provided with auniform dispersion of second phase particles of submicron and largersize. The molten host metal may also be the same as the solvent metalmatrix of the porous composite, but of so great a volume, as compared tothe porous composite, that in-situ second phase precipitation would bedifficult to effect or control. The concentration of the second phase inthe porous composite need not be large, however.

It is believed that the prior art suggestions of introduction of finesecond phase particles directly to a molten metal bath are technicallydifficult and produce metal products having less desirable propertiesupon solidification due to a deleterious layer, such as an oxide, whichforms on the surface of each second phase particle at the time of orprior to introduction into the molten metal bath. The second phaseparticles of the present invention, being formed in-situ, do not possessthis deleterious coating or layer. Thus, the present invention may leadto metal products having unexpectedly superior properties.

Three basic reaction modes to make porous composite have been identifiedin accordance with the present invention. In the first mode, thestarting materials constitute individual powders of each of the solventmetal and the individual constituents of the second phase to be formed.For example, a mixture of aluminum, titanium, and boron may be compactedinto a rod and ignited locally to cause an isothermal, propagatingreaction wave front to consume the elements and to form a dispersion oftitanium diboride in an aluminum matrix.

In the second mode of the invention, individual alloys may be reacted,one such alloy comprising an alloy of the solvent metal with one of theconstituents of the second phase, and the other comprising an alloy ofthe same solvent metal, or another metal with which the solvent metalreadily alloys, with the other constituent of the second phase. As anexample of using two alloys of a common metal, a mixture ofaluminum-titanium alloy with aluminum-boron alloy may be compacted intoa rod and ignited locally to cause an isothermal, propagating reactionwave front to consume the elements and to form a dispersion of titaniumdiboride in aluminum. This alloy-alloy reaction route may, in somecases, be relatively slower than the elemental route, yet may offereconomic advantages because the alloys utilized can be cheaper than theelemental powders.

The third reaction mode constitutes a combination, or intermediate, ofthe first two modes discussed above. Thus, one may react a premixedalloy containing one reactive species and the solvent matrix metal, withan elemental powder of the second reactive species, such as combining analuminum-titanium alloy with elemental boron powder, compacting into arod, and igniting locally to cause an isothermal, propagating reactionwave front to consume the elements and to form a dispersion of titaniumdiboride in an aluminum matrix. This reaction mode may be relativelymore expensive than the alloy-alloy reaction mode, but offers a morerapid reaction, which in turn permits formation of finer particleprecipitates than obtainable by the alloy-alloy route. However, thealloy-elemental powder reaction mode could be relatively less expensive,although slower, than the elemental powder mode, in most cases.

It should be noted in performing the process of the present inventionthat certain criteria must be met in order to produce the desired porouscomposite. Firstly, the heat generated by the initial local reaction ofthe second phase-forming constituents must be sufficient to allow thereaction wave front to propagate through the reaction mass. In addition,the heat source, such as inductively heated graphite, should supplysufficient local heat to initiate the second phase-forming reaction by,for example, locally melting solvent metal.

Both of the above criteria have a significant impact on the feasibilityof different composite forming reactions performed in accordance withthe present invention because the relatively high volume fractions ofsolvent metal in the reaction mass absorb heat and therefore tend toquench the reaction. For this reason, it can be important to preheat thereactant mass prior to local initiation of reaction. Preheating may thuspermit certain non-propagating reactions to propagate, or, in thealternative, allow reactions to propagate at higher solvent metalconcentrations. Other advantages to preheating include the ability toremove adsorbed gases from the reaction mass prior to initiation, andthe attainment of higher maximum reaction temperatures that permit thesecond phase-forming reaction to go substantially to completion.

The resultant intermediate composite from any of the reaction modesmentioned above, typically a porous concentrate, may be subsequentlycombined with additional metal in the admixture procedure. As describedearlier, this procedure yields dense composites with superior propertiesthat combine the beneficial effects of in-situ precipitated dispersoidsand molten metal processing to achieve the required loading of secondphase. In one embodiment of the present invention the in-situ secondphase formation process and the admixture procedure are performedsequentially without a break, or, in the alternative, are made to occuralmost simultaneously. Such a procedure has obvious advantages in thatintermediate materials handling operations are eliminated, and just oneapparatus may be used for both processes. Typical of such a combinedsecond phase formation and admixture procedure would be the preparationof a compacted rod of elemental boron, titanium and aluminum powders,followed by suspending the rod in such a manner as to dip the end of therod into a bath of molten aluminum. The self propagating, substantiallyisothermal reaction could then be allowed to consume the secondphase-forming constituents before the reacted compact is admixed withthe molten metal by releasing the suspension means. Alternatively, therod could be immersed into the molten metal essentially concurrentlywith the second phase-forming reaction by more rapid release of thesuspension means. In the extreme, the rod might be replaced withcompacted briquettes, for example, that are simply dropped into themolten metal. Accordingly, the self-propagating, substantiallyisothermal reaction might take place in the briquette while submerged inthe melt, thereby causing essentially simultaneous formation anddispersion of second phase dispersoids.

It is particularly to be noted that the prior art teaches that thecombination of elemental metal powders, or alloy powders, particularlyof a coarse particulate size, would typically yield intermetalliccompounds. In fact, conventional techniques for forming intermetallicsinvolve, for example, reacting a mixture of titanium and aluminum, toform titanium aluminide, and a mixture of boron and aluminum to formaluminum diboride. Thus, one would expect that a mixture comprisingpowders of titanium, aluminum, and boron would yield an aggregateagglomeration of titanium aluminide, aluminum diboride, and possibly,titanium diboride. In contrast, the present invention provides for theformation of essentially just one finely dispersed precipitate from thetwo reactive components in a matrix of the third component. It isimportant that the second phase precipitate material not be soluble inthe solvent metal, while the constituents of the second phase,individually, are at least sparingly soluble in the solvent metal. Thus,the exothermic dispersion reaction mechanism depends upon a certainamount of each second phase-forming constituent dissolving and diffusingin the solvent metal, and while in solution (either liquid or solidstate), reacting exothermically to form the insoluble ceramic, whichprecipitates rapidly as a fine particulate. The solvent metal provides amedium in which the reactive elements may diffuse and combine. Once theinitial reaction has occurred, the heat released by the exothermicreaction may cause additional solvent metal to melt, thereby enhancingdiffusion of reactive components in the solvent metal, and completingthe reaction.

The cool-down period following initiation of the reaction andconsumption of the reactive constituents is believed important toachieving very small particle size, and limiting particle growth. It isknown that at high temperatures, it is possible for the second phaseparticles to grow, or sinter together. This should also be avoided, inmost cases, because of the negative effect of large particle sizes onductility. The cool-down or quenching of the reaction is, in a sense,automatic, because once the second phase-forming constituents arecompletely reacted, there is no further energy released to maintain thehigh temperatures achieved. However, one may control the rate ofcool-down to a certain extent by control of the size and/or compositionof the mass of material reacted. That is, large thermal masses absorbmore energy, and cool down more slowly, thus permitting growth of largerparticles, such as may be desired for greater wear resistance, forexample, for use in cutting tools. Fast cooling may be achieved, forexample, by placing the reaction mass on a water-cooled coppersubstrate. This avoids the contamination typically obtained withrefractory substrates such as alumina.

Initiation of the reaction is accomplished by local heating of a portionof the reaction mass, rather than heating of the entire mass. Localizedheating may be achieved by electrical impulse, thermite spark, laser,etc. The preferred method is inductive heating of a graphite susceptor.

While it is unnecessary to actually reach the melting temperature toinitiate the reaction, a temperature where localized melting occurs mustbe achieved, or where substantial diffusion of the reactive species inthe solvent metal can occur. In some cases, as temperature increases itis possible for the starting constituents to diffuse into the solventmatrix metal, forming an alloy therewith having a lower meltingtemperature than the matrix metal. Thus, reaction initiation temperatureis lowered.

Regarding impurities, the solvent metal may be alloyed in conventionalmanner, while in the reactive constituents, large amounts of alloyingelements or impurities may cause problems in certain instances. Forexample, the presence of large amounts of magnesium in boron may inhibitthe formation of titanium diboride in an aluminum matrix by forming amagnesium-boron complex on the surface of the boron particles, thuslimiting diffusion of the boron in the matrix. However, the presence ofmagnesium in the aluminum does not have this effect. That is, borideforming materials in the boron itself may inhibit the desireddissolution or diffusion of the boron and its subsequent reaction toform titanium diboride. Likewise, thick oxide films around the startingconstituent powders may also act as barriers to diffusion and reaction.Extraneous contaminants, such as absorbed water vapor, may also yieldundesirable phases such as oxides or hydrides, or the powders may beoxidized to such an extent that the reactions are influenced.

It is noted that undesirable compounds which may be formed from thereaction of one constituent and the solvent metal during the porouscomposite formation process can be essentially eliminated in someinstances by the addition of more of the other constituent. For example,titanium aluminide formation in the titanium diboride-aluminum porouscomposite may be substantially eliminated by adding boron abovestoichiometric proportion prior to initiation of the secondphase-forming reaction. The boron can be in the form of elemental boron,boron alloy or boron halide. It is also noted that in the admixtureprocess, wherein composite material of the present invention is added toa molten host metal, undesirable compounds formed in the compositematerial by the reaction of one constituent and the solvent metal may beintroduced into the melt. These undesirable compounds may be essentiallyeliminated by adding an additional amount of another constituent to themolten host metal. For example, titanium aluminide formed in a titaniumdiboride-aluminum composite material may be essentially removed from ahost aluminum melt by adding additional boron to the melt. Such a boronaddition also provides the benefit that any free titanium, which canadversely affect the viscosity of the melt for casting operations isconverted to titanium diboride.

It is also to be noted that, in accordance with the present invention,the complex precipitation of a plurality of systems may be caused. Thus,it is possible to precipitate complex phases, such as Ti(B₀.5 C₀.5), oralternatively, to precipitate plural second phases, such as a mixture oftitanium diboride and zirconium diboride in an aluminum matrix, inaccordance with the reaction:

    Ti+Zr+4B+Al→TiB.sub.2 +ZrB.sub.2 +Al.

Substitution of titanium by zirconium or vice versa, is also possible,yielding complex borides of the type (Ti,Zr)B₂.

It is also possible to achieve a low temperature solvent assistedreaction in a metal matrix which has a high melting temperature byallowing or admixing the high melting metal with a lower melting solventmetal. This may allow for easier initiation and propagation.

In accordance with the present invention, it has been found that thepowders need not be compacted prior to localized firing, but doing soallows easier diffusion and thus easier initiation. This is due tolocalized melting, and increased diffusion, which are possible when thepowders are in close proximity.

The starting powders must be protected from extensive oxidation due toexposure to the atmosphere, as this will restrict the diffusion of thecomponents into the solvent metal matrix, and the reaction shouldpreferably be carried out under an inert gas to minimize oxidation athigh temperatures.

In accordance with the present method, particle growth of the secondphase can be controlled. As is known in the art, the elevatedtemperatures produced as, for example, by the exothermic reaction, willremain higher and subside more slowly for a large mass of material thanfor a smaller mass. These conditions of high temperature for longperiods of time favor particle growth of ceramics. Thus, the formationof relatively small volume porous composites of in-situ formed ceramicwill facilitate quicker cooling and limit particle growth of the ceramicphase.

The particle size of the second phase reaction product is dependent uponheat-up rate, reaction temperature, cool-down rate, crystallinity andcomposition of the starting materials. Appropriate starting powder sizesmay range from less than 5 microns to more than 200 microns. Foreconomic reasons, one may normally utilize larger partle size powders.It has been found that the particle size of the precipitated secondphase in the matrix may vary from less than about 0.01 microns to about5 microns or larger, dependent upon such factors as those discussedabove.

It has been found that some specific reactant properties have a greaterimpact than powder particle size on the particle size of the secondphase produced. For example, the use of amorphous boron may result inthe precipitation of a finer particle size titanium diboride than doesthe use of crystalline boron in an otherwise comparable mixture. Theprecipitation of specific particle size second phase may be selectivelycontrolled by proper control of starting composition, temperature ofreaction, and cool-down rate.

In selecting the constituents and the solvent matrix metal for thecomposite materials produced by the above-described process, it isimportant that the formed second phase material have a low solubility inthe molten mass, for example, a maximum solubility of 5 weight percent,and preferably 1 percent or less, at the temperature of the molten hostmetal. Otherwise, significant particle growth in the second phasematerial may be experienced over extended periods of time. For most usesof composite materials, the size of the second phase particles should beas small as possible, and thus particle growth is undesirable. When thesolubility of the formed second phase material in the molten mass islow, the molten mass with dispersed second phase particles can bemaintained in the molten state for a considerable period of time withoutgrowth of the second phase particles. For example, a molten mass ofaluminum containing dispersed titanium diboride particles can bemaintained in the molten state for three to four hours withoutappreciable particle growth.

One advantage of the admixture process is that the use of porouscomposite, particularly that having a high loading of second phasematerial, permits one to simply make a single batch of porous compositematerial. One may then produce a wide variety of final composites havingdifferent second phase loadings. Additionally, with the admixtureprocedure, it is possible to form the second phase material in a matrixmetal which is conductive to the formation of particles of a desiredtype, size, and morphology, and thereafter incorporate the particles ina host metal in which such particles cannot otherwise be produced.

A further advantage of the use of the admixture concept is the fact thatin the in-situ precipitation of second phase material in a solvent metalmatrix, the particle size of the second phase material appears to berelated to the loading level of the second phase material. For example,in titanium diboride-aluminum composites, particle size decreases withhigher concentration, up to about 40-60 percent second phase material,and then the particle size increases as the concentration approaches 100percent. Thus, for example, if the smallest possible particle size wasdesired in a final composite having a low second phase concentration,one could prepare a second phase-containing concentrate in the 40-60percent concentration range of titanium diboride to yield the smallestparticles possible, and thereafter admix the porous composite to thedesired second phase concentration.

Thus, according to the invention, the weight concentration of thesolvent metal exceeds 10 volume percent, more preferably exceeds 20volume percent, and most preferably more than 30 percent. Also, theporosity of the product exceeds 1 percent, more preferably exceeds 10percent, and most preferably exceeds 25 percent. The particle size ofthe dispersoid particles may vary from about 0.01 microns to about 5microns, preferably from 0.1 microns to about 3 microns.

The reactants may be formed into any desired conventional shape.Typically, a rod or cylindrical green compact is used. The shapedarticle can be compressed in a manner known in the art. Any shape isuseful that facilitates local ignition. In a conventional manner, an endof a rod is ignited and the isothermal wave front moves along the rod toits terminal end. Any conventional means for ignition can be used.

Examples 1 through 6 illustrate the production of titanium diboridesecond phase particles in aluminum matrices and the effects ofatmosphere, compaction pressure, and preheating on reaction propagationrate.

EXAMPLE 1

Titanium, boron, and aluminum powders are ball-milled in the properstoichiometric proportions to provide 60 weight percent titaniumdiboride second phase in an aluminum solvent matrix. The mixture is thenpacked in gooch tubing and isostatically pressed to 40 ksi, forming acompact approximately 1 centimeter in diameter by 5 centimeters long andhaving a density of 2.39 grams per cubic centimeter. The compact is thenplaced end to end with a graphite rod in a quartz tube under flowingargon. The graphite rod is heated in a radio frequency field whichinitiates a reaction at the interface of the compact and the rod. Thereaction propagates the length of the compact at a rate of 0.77centimeters per second. Analysis of the resultant composite mateialreveals a dispersion of substantially unagglomerated titanium diborideparticles having an average diameter of approximately 1 micron in analuminum matrix.

EXAMPLE 2

A compact containing titanium, boron, and aluminum is prepared andreacted as in Example 1, with an additional step of preheating thecompact to 500° C. prior to initiation of the reaction. The reaction isobserved to propagate faster than in the unpreheated compact at a rateof 1.38 centimeters per second.

EXAMPLE 3

A compact containing titanium, boron, and aluminum is prepared andreacted as in Example 2, except that the reaction is done in a vacuumrather than under flowing argon. The reaction propagates at 1.33centimeters per second.

EXAMPLE 4

A compact containing titanium, boron, and aluminum is prepared andreacted as in Example 1, except that the reaction is done in anatmosphere of flowing helium rather than argon. The reaction is observedto propagate at a rate of 0.47 centimeters per second.

EXAMPLE 5

A compact is prepared and reacted as in Example 1, except that themixture of titanium, boron and aluminum powders is compacted to 13 ksirather than 40 ksi, yielding a compact having a lower density of 2.06grams per cubic centimeter. The reaction propagates at 0.66 centimetersper second.

EXAMPLE 6

A compact containing titanium, boron, and aluminum is prepared andreacted as in Example 5, except that the reaction is done in a vacuumrather than under flowing argon. The reaction is observed to propagateat a rate of 0.44 centimeters per second.

The following example illustrates the ability to produce a compositematerial comprising titanium carbide second phase particles in analuminum matrix by the process of the present invention and thesubsequent addition of the composite material to molten aluminum toproduce a composite of lower second phase loading.

EXAMPLE 7

239.5 grams of titanium powder, 60.3 grams of carbon black, and 200.2grams of aluminum powder are ball-milled for 30 minutes, packed in goochtubing, and isostatically pressed to 40 ksi, forming a green compact 1inch in diameter by 12 inches long. The compact is placed on two watercooled copper rails in a 4 inch diameter quartz tube under flowingargon. A 1 inch by 1 inch piece of carbon placed next to one end of thecompact is induction heated until an exothermic reaction is initiated atthe end of the compact. Power to the induction unit heating the carbonis turned off and the reaction is allowed to propagate the length of thecompact. When cool, the reacted concentrate, comprising 60 weightpercent titanium carbide second phase particles in an aluminum matrix,is crushed and slowly added to molten aluminum at 770° C. whilemechanically stirring. The meltis maintained at 770° C. and stirredvigorously for several minutes. The melt is then fluxed with chlorinegas for 15 minutes, skimmed, and cast. The resultant material containsapproximately 7.5 volume percent titanium carbide second phase particlesin an aluminum matrix.

The following example illustrates the production of a composite materialcomprising titanium diboride second phase particles in an aluminummatrix by the process of the present invention, including the use ofboron above stoichiometric proportion. The example also demonstrates thesubsequent introduction of this composite material into additionalaluminum to produce a composite of lower second phase loading.

EXAMPLE 8

207 grams of titanium powder, 106 grams of boron powder (15 weightpercent above stoichiometric proportion), and 200.2 grams of aluminumpowder are ball-milled for 30 minutes, packed in gooch tubing, andisostatically pressed to 40 ksi, forming a green compact approximately 1half inch in diameter by 12 inches long. The compact is placed on awater cooled copper trough in a 2 inch diameter quartz tube underflowing argon. A 1 inch by 1 inch piece of carbon placed next to one endof the compact is induction heated until an exothermic reaction isinitiated at the end of the compact. Power to the induction unit heatingthe carbon is turned off and the reaction is allowed to propagate thelength of the compact. When cool, the reacted concentrate is crushed andslowly added to molten aluminum at 770° C. while mechanically stirring.The melt is maintained at 770° C. and stirred vigorously for severalminutes. The melt is then fluxed with chlorine gas for 15 minutes,skimmed, and cast. The resultant material contains approximately 10volume percent titanium diboride second phase particles having anaverage size of 0.9 microns in an aluminum matrix, substantially free oftitanium aluminide.

It is noted that the present invention has a number of advantages overmethods taught by the prior art. For example, this invention circumventsthe need for submicron, unagglomerated refractory metal boride startingmaterials, which materials are not commercially available, and are oftenpyrophoric. Further, the present invention yields a porous compositewith a second phase precipitated therein, suitable for admixture with ahost metal to achieve a final composite having superior hardness andmodulus qualities over currently employed composites, such asSiC/aluminum. This admixture process also eliminates the technicalproblems of uniformly dispersing a second phase in a molten metal, andavoids the problem of oxide or other deleterious layer formation at thesecond phase/metal interface during processing. Final metal matrixcomposites prepared from the porous composites of the present inventionalso have improved high temperature stability, in that the second phaseis not reactive with the metal matrix. Further, such final metal matrixcomposite can be remelted and recast while retaining fine grain size,fine particle size, and the resultant superior physical properties.

It is understood that the above description of the present invention issusceptible to considerable modification, change, and adaptation bythose skilled in the art, and such modifications, changes, andadaptations are intended to be considered to be within the scope of thepresent invention, which is set forth by the appended claims.

We claim:
 1. A porous mass comprising a substantially uniform dispersionof up to about 80 volume percent in-situ precipitated second phaseparticles in a solvent metal matrix, said mass produced by propagatingat or below atmospheric pressure, a substantially isothermal reactionwave front of second phase-forming constituents in the presence of atleast about 20 volume percent of solvent metal in which saidconstituents are more soluble than said second phase.
 2. The dispersionas set forth in claim 1, wherein the second phase particles comprisefrom about 40 to about 60 percent by weight of the composite formed. 3.The dispersion as set forth in claim 2, wherein the particle size of thesecond phase is from about 0.1 to about 3 microns.
 4. The dispersion asset forth in claim 1, wherein the second phase is titanium diboride andthe solvent metal matrix is aluminum or an alloy thereof.
 5. Thedispersion as set forth in claim 1, wherein the second phase is titaniumcarbide and the solvent metal matrix is aluminum or an alloy thereof. 6.The dispersion as set forth in claim 1, wherein the second phase istitanium nitride and the solvent metal matrix is aluminum or an alloythereof.
 7. The porous mass as set forth in claim 1, wherein theporosity exceeds about 10 percent.
 8. The porous mass as set forth inclaim 1, wherein the porosity exceeds 25 percent.
 9. The dispersion asset forth in claim 1, wherein the concentration of the solvent metalexceeds 20 percent by volume.
 10. The dispersion as set forth in claim1, wherein the concentration of the solvent metal exceeds 30 percent byvolume.
 11. The dispersion as set forth in claim 1, wherein the secondphase particles have a particle size between about 0.1 and 3 microns.12. The dispersion as set forth in claim 1, wherein the second phaseparticles are substantially encapsulated by solvent metal.
 13. Thedispersion as set forth in claim 1, wherein the second phase particlesinclude titanium diboride or titanium carbide and the solvent metalmatrix includes aluminum or an alloy thereof.